Driver amplifier

ABSTRACT

The disclosure relates to a driver amplifier circuit. The driver amplifier circuit includes a non-linear differential amplifier and a non-linear resistor connected across output terminals of the differential amplifier. The non-linear resistor has a resistance value that increases as the differential voltage amplitude across the non-linear resistor increases. A transmitter may include the driver amplifier.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/EP2019/077236, filed on Oct. 8, 2019, the disclosure of which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present application relates to driver amplifier circuit and inparticular, but not exclusively, to a driver amplifier circuit for highspeed optical communications systems.

BACKGROUND

In transmitters for optical communications, a differential driveramplifier is used to increase the signal level generated from ahigh-speed digital signal source in order to properly drive anelectro-optical modulator.

In order to increase data-rates of optical transmitters, quadratureamplitude modulation (QAM) schemes have been recently introduced inoptical communications resulting in much more stringent requirements fortotal harmonic distortion (THD) of driver amplifier.

Accordingly, any solution that allows a driver amplifier to be providedwith an improved THD is of interest.

SUMMARY

In a first aspect there is provided a driver amplifier circuitcomprising a non-linear differential amplifier and a non-linear resistorconnected across output terminals of the differential amplifier, whereinthe non-linear resistor has a resistance value that increases as thedifferential voltage amplitude across the non-linear resistor increases.

The non-linear resistor counteracts the non-linearity of thedifferential amplifier by having a resistance that increases withvoltage. As the output current of the driver amplifier circuit will bethe difference between the current output of the differential amplifierand the current through the non-linear resistor, the non-linearity inthe output current may be reduced or eliminated thereby reducing thetotal harmonic distortion (THD) of the driver amplifier circuit.

Further such a circuit may be constructed so as to have a low parasiticcapacitance and therefore a wide bandwidth. Also, the addition of thenon-linear resistor does not necessitate any additional biasing and thusthe THD in the circuit can be reduced without requiring additional powerconsumption.

In an implementation of the first aspect, the non-linear resistor isconfigured to absorb a current that depends on voltage according to ahyperbolic tangent function. Accordingly, for small voltage levels thecurrent will grow linearly but for higher voltages the current willsaturate, thus generating higher order harmonics which will cancelcorresponding harmonic content in the differential amplifier signal.

In an implementation of the first aspect, the non-linear resistor isconfigured so that the current flowing across the non-linear resistor issubtracted from the output current of the non-linear differentialamplifier to provide a current at the output of the driver amplifiercircuit that is substantially linear with respect to an input voltageprovided at inputs of the non-linear differential amplifier. Thesubtraction provides the cancellation of the harmonics and providing alinear signal response for the driver amplifier circuit.

In an implementation of the first aspect, the non-linear resistorcomprises at least a first transistor configured to operate as atwo-terminal device. A transistor such as an FET provides a simple andreadily available component to provide the non-linearity required of thenon-linear resistor. For example, where the transistor is a FET (e.g. aMOSFET), the gate may be connected to the source or the drain therebypermitting the FET to operate passively (i.e. not requiring separatebiasing) as a non-linear resistive element. The non-linear resistor mayinclude second transistor configured as a two-terminal device that isconnected in series with the first transistor. In an implementation, thefirst and second transistors are field effect transistors. The gate ofthe first transistor may be coupled to a gate terminal of the secondtransistor. This is advantageous because the gate of the respectivetransistors does then not need to be connected to the output terminals(rails) of the amplifier and the parasitic capacitance associated withthe gate can be eliminated or reduced.

In an implementation of the first aspect, the drain terminal of thefirst transistor is coupled to a positive output terminal of thedifferential amplifier, the drain terminal of the second resistor iscoupled to a negative output terminal of the differential amplifier, andthe source terminal of the first and second transistors is coupled tothe gate terminals of the first and second transistors. In anotherimplementation, the source terminal of the first transistor is coupledto a positive output terminal of the differential amplifier, the sourceterminal of the second resistor is coupled to a negative output terminalof the differential amplifier, and the drain terminal of the first andsecond transistors is coupled to the gate terminals of the first andsecond transistors. In another implementation, the drain terminal of thefirst transistor is coupled to a positive output terminal of thedifferential amplifier, the source of the second resistor is coupled toa negative output terminal of the differential amplifier, and the drainterminal of the first transistor and the source terminal of the secondtransistor are coupled with the gate terminals of the first and secondtransistors.

In an implementation of the first aspect, the driver amplifier circuitfurther comprises a linear resistor connected between the non-linearresistor and the positive and/or negative output terminals of thedifferential amplifier. By including a resistor between the transistorterminal and the output terminal(s) (e.g. rail(s)) of the differentialamplifier, the circuit can be made resistant to damaging high currentscaused by unexpected electrostatic discharge, for example, when thecircuit is being installed or inspected.

In a second aspect, there is provided a transmitter comprising a driveramplifier circuit according to any implementation of the first aspect.In an implementation of the second aspect, the transmitter may comprisean electro-optical modulator configured to modulate an optical signalaccording to the output of the driver amplifier circuit. Accordingly, amodulated optical signal with low THD may be provided using a powerefficient and high bandwidth driver. In an example, the transmitter isan optical module or another optical networking device.

In an implementation of the second aspect, the transmitter may comprisea signal source configured to provide a signal to the driver amplifiercircuit. The signal source may be configured to provide a signalmodulated according to a quadrature amplitude modulation scheme. Forexample, in an implementation, the signal source may comprise adigital-to-analogue converter. Thus, data for transmission can beprovided as an analogue signal which may be effectively amplified by thedriver amplifier circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 shows a block diagram showing components of a transmitter for acommunications system;

FIG. 2 shows a block diagram showing components of a transmitteraccording to a communications system;

FIG. 3 shows a schematic block diagram showing components of atransmitter according to an embodiment of the invention;

FIGS. 4a, 4b and 4c show current-voltage plots corresponding to adifferential amplifier, a non-linear differential resistor (NDR) and atan output from a driver circuit respectively, according to an embodimentof the invention;

FIG. 5 shows a block diagram of a transmitter according to an embodimentof the invention in which the NDR consists of a field effect transistor;and

FIG. 6 shows a block diagram of a transmitter according to an embodimentof the invention in which the NDR consists of a pair of field effecttransistors connected in series;

FIG. 7 shows a block diagram of a transmitter according to an embodimentof the invention where the NDR consists of a pair of field effecttransistors connected in series;

FIG. 8 shows a block diagram of a transmitter according to an embodimentof the invention where the NDR consists of a pair of field effecttransistors connected in series;

FIG. 9 shows a block diagram of a transmitter according to an embodimentof the invention where the NDR consists of a pair of field effecttransistors connected in series where the field effect transistors arecoupled to output rails of a differential amplifier via respectiveresistors.

DESCRIPTION

Example embodiments are described below in sufficient detail to enablethose of ordinary skill in the art to embody and implement the systemsand processes herein described. It is important to understand thatembodiments can be provided in many alternate forms and should not beconstrued as limited to the examples set forth herein.

Accordingly, while embodiments can be modified in various ways and takeon various alternative forms, specific embodiments thereof are shown inthe drawings and described in detail below as examples. There is nointent to limit to the particular forms disclosed. On the contrary, allmodifications, equivalents, and alternatives falling within the scope ofthe appended claims should be included. Elements of the exampleembodiments are consistently denoted by the same reference numeralsthroughout the drawings and detailed description where appropriate.

The terminology used herein to describe embodiments is not intended tolimit the scope. The articles “a,” “an,” and “the” are singular in thatthey have a single referent, however the use of the singular form in thepresent document should not preclude the presence of more than onereferent. In other words, elements referred to in the singular cannumber one or more, unless the context clearly indicates otherwise. Itwill be further understood that the terms “comprises,” “comprising,”“includes,” and/or “including,” when used herein, specify the presenceof stated features, items, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, items, steps, operations, elements, components, and/orgroups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein are to be interpreted as is customary in the art. Itwill be further understood that terms in common usage should also beinterpreted as is customary in the relevant art and not in an idealizedor overly formal sense unless expressly so defined herein.

An optical transmitter system 100 is shown in FIG. 1 which includes asignal source 101, a driver amplifier 102 and an electro-opticalmodulator 103. The source 101 may include a digital-to-analogueconverter (DAC). This may be configured to receive digital signalcomprising a binary data stream and encode the binary data stream assymbols in an analogue signal according to an existing modulationscheme. For example, a quadrature phase shift keying (QPSK e.g. 8-PSK)or quadrature amplitude modulation (QAM, e.g. 16-QAM, 32-QAM or 64-QAM)scheme may be used to generate the analogue signals from high speeddigital signal source. Even simpler schemes such as return-to-zero (RZ)or non-return-to-zero (NRZ) and on/off keying (OOK) may alternatively oradditionally be used. The scheme chosen may depend on the spectralefficiency required and the complexity of the source providing thesignal for modulation by the electro-optical modulator 103.

The driver amplifier 102 is an electrical signal amplifier which isconfigured to receive analogue electrical signals from the signal source101 and amplify them to a level that is suitable for causing modulationat the electro-optical modulator 103. A typical driver amplifier will bea differential amplifier with differential inputs and outputs (i.e. afully differential amplifier).

The electro-optical modulator 103 uses the electro-optical effect tomodulate the phase of light travelling through the modulator. Forexample, light may travel through a crystal (e.g. lithium niobiate)having a refractive index which depends on the strength of the localelectric field. The local electric field may be modulated by the signaloutput by the driver 102 to modulate the light. In an embodiment, theelectro-optical modulator 103 may include one or more Mach-Zehnderelectro-optical modulators for suitably modulating the I and Qcomponents, for example, for transmission of a QPSK or QAM signal. Othertypes and topologies, technologies and materials of electro-opticalmodulator are possible as will be appreciated by those skilled in theart.

The transmitter may be embodied, for example, in any opticalcommunications device in an optical network. One example of an opticalcommunications device is an optical module. An optical module is ahot-pluggable transmitter (or transceiver) used in high bandwidth datacommunications applications (e.g. greater than 10 Gbit/s).

A scheme adopted to reduce the THD of a driver amplifier 102 is shown inFIG. 2. The driver amplifier 102 has differential input terminals (IN+and IN−) connected to the outputs of the source 101 and differentialoutput terminals (OUT+ and OUT−) connected to the electro-opticalmodulator 103. The driver amplifier 102 is composed of two circuitelements, a main driver 201 and a secondary driver 202. A differentialinput signal from the source 101 is split between differential inputs(MD_IN+ and MD_IN−) of the main driver 201 and differential inputs(SD_IN+ and SD_IN−) of the secondary driver 202

The differential outputs of the main driver (MD_OUT+ and MD_OUT−) and ofthe secondary driver (SD_OUT+ and SD_OUT−) are connected in oppositephase, i.e. the positive output terminal of the main driver MD_OUT+ isconnected to the negative output terminal of the secondary driverSD_OUT− and the negative output terminal of the main driver MD_OUT− isconnected to the positive output terminal of the secondary driverSD_OUT+. The main driver 201 is designed to linearly amplify the inputsignal at the operating frequency ‘f’. However, due to the THD of themain driver 201, it will also produce its higher order harmonics (‘2f,3f . . . ’), that have generally a lower amplitude with respect to theoperating frequency ‘f’.

The secondary driver 202 is designed to behave as strongly nonlinear. Asa consequence, for a small amplitude of operating frequency component‘f’, higher amplitude signal components for higher order harmonics (‘2f3f . . . ’) are generated. Since the outputs MD_OUT+, MD_OUT− of themain driver 201 and SD_OUT+, SD_OUT− of the secondary driver areconnected in opposite phase, the result is that both the components atthe operating frequency f and the higher order harmonic components (‘2f3f . . . ’) of the signal output from the secondary driver 202 aresubtracted from the signals output by the main driver 201. As theamplitude of ‘f’ of the main driver is much higher than the amplitude ofT of the signal output by the secondary driver, the final amplitude ofthe operating frequency component ‘f’ that reaches the electro-opticalmodulator is only slightly smaller than the signal component atfrequency T generated by the main driver 201. At the same time, giventhe amplitudes of the higher order harmonics (‘2f 3f . . . ’) outputtedfrom the main driver are similar to the amplitudes of the higher orderharmonics (‘2f 3f . . . ’) outputted from the secondary driver, thefinal amplitudes of the higher order harmonics (‘2f 3f . . . ’) thatreach the electro-optical modulator is close to zero. The result is thatthe THD level of the signals that are provided to the input of theelectro-optical modulator are very low.

There are a number of disadvantages with this approach, however. Theseinclude:

-   -   1) Higher power consumption: the secondary driver 202 requires        biasing and, thus, consumes DC power thereby increasing the        overall power consumption of the driver amplifier circuitry 102;    -   2) Reduced bandwidth: the inclusion of a secondary driver 202        increases the input and output parasitic capacitances of the        driver, thus reducing the maximum operating frequency of the        driver amplifier circuitry 102. This can make it unsuitable for        high data rate applications.

In summary, the scheme described in FIG. 2 may improve the THD, but at acost of higher power consumption and reduced operating bandwidth.

An embodiment is shown in FIG. 3, whereby a driver amplifier 102 isshown which uses a non-linear differential resistor (NDR) 302 toeffectively linearize the output of the driver amplifier and provide anoutput signal to the electro-optical modulator 103 with low THD.

Specifically, as shown in FIG. 3, the driver amplifier has a singledifferential amplifier as a driver 301. A differential input signal isprovided by the source 101 at inputs IN+ and IN− of the driver 301 whichprovides a differential output at positive and negative output rails(terminals) 301 a and 301 b respectively. In other words, the driver 301may be a fully differential amplifier according to embodiments. Anonlinear differential resistor (NDR) is added at the output of thedriver 301 between the output terminals i.e. across the output rails 301a, 301 b of the driver 301. The differential signal (OUT+, OUT−) acrossthe NDR is provided at the output of the driver amplifier 102 to theelectro-optical modulator 103. As shown a driver current I_(DRIVER)flows between the terminals of the driver 301. A portion of that drivercurrent I_(DRIVER) will pass through the NDR as I_(NDR) and theremainder will be the output current TOUT which flows through theelectro-optical modulator as the differential signal outputs. The mainbehavior or characteristic of the nonlinear differential resistor isthat the resistance of the NDR increases with increasing voltage droppedacross its terminals 302 a, 302 b.

The functional behavior of the circuit of FIG. 3 can be explained byreference to the current-voltage characteristics shown in FIGS. 4(a),(b) and (c). The differential output current of the driver (I_(DRIVER))has a behavior 401 shown in FIG. 4(a), where the horizontal axisrepresents the differential input voltage (V_(IN)=V_(IN)+−V_(IN)−) andthe vertical axis the current I_(DRIVER). The current I_(DRIVER) growssubstantially linearly with respect V_(IN) for small amplitude levels ofV_(IN), then I_(DRIVER) begins to saturate at higher amplitude levelsbecause higher order harmonics are generated. The current flowing acrossthe nonlinear differential resistor NDR (I_(NDR)) has the behavior 402shown in FIG. 4(b). As for the driver current I_(DRIVER), the currentI_(NDR) also grows substantially linearly with respect to the inputvoltage V_(IN) across its terminals for small amplitude levels ofV_(IN), then it saturates because higher order harmonics are generated.The final differential output current that reaches the electro-opticalmodulator (I_(OUT)) is the difference between I_(DRIVER) and I_(NDR),resulting in the current-voltage characteristic 403 shown in FIG. 4(c).As shown, the output current I_(OUT) is substantially linear withrespect to input voltage V_(IN), resulting in a very low level of THD.In other words, by having an NDR 302 connected across the outputterminals of the driver (differential amplifier) 301, the currentflowing across the NDR 302 I_(NDR) is subtracted from the output currentat the differential amplifier 301 to provide a current output of thedriver amplifier circuit 102 that is substantially linear with respectto input voltage V_(in).

By designing the NDR 302 response appropriately, therefore, thenon-linearity in the driver output current may be negated and the outputof the driver amplifier 102 linearized to provide a very low THD. Thecurrent-voltage characteristic of the NDR 302 may be considered as ahyperbolic tangent function according to embodiments. By selecting ordesigning appropriate circuitry to approximate or replicate thisbehavior the desirable cancellation of the harmonic distortion presentin the driver 301 can be achieved. Moreover, using such a nonlineardifferential resistor does not require bias current and voltage, as aconsequence the overall power consumption is not increased. Further, theinput/output parasitic capacitances of the non-linear differentialresistor circuitry 302 may be designed to be much lower than a driversuch as a differential amplifier, as a consequence the bandwidth of thedriver amplifier circuit 102 is not negatively affected. Accordingly,the use of an NDR 302 allows driver amplifier circuits 102 with very lowTHD level to be provided without increasing power consumption and/orreducing operating bandwidth.

Various embodiments will now be described which show electronic circuitelements or combinations of circuit elements which would provide therequired NDR 302 behaviour. The invention is not limited to theseembodiments, however, and it is envisaged other circuitry may be usedwhich provides the behaviour of resistance increasing with voltage e.g.according to a hyperbolic tangent function, or other function whichprovides a characteristic that negates at least a portion of the THD ofthe driver 301.

FIG. 5 shows an embodiment of the driver amplifier circuit 102 in whichthe NDR is implemented using a field effect transistor (FET) 501. Thegate and the drain of the FET are coupled together such that the FEToperates as a two-terminal resistive device. The non-linearity of theFET response when configured as a resistor in this manner may providethe required NDR characteristic. The coupled gate (G) and drain (D)terminals are connected to the negative differential output rail 303 b,while the source (S) is connected to the positive output rail 303 a.

As the transistor behaves symmetrically when configured as atwo-terminal device the FET 501 could, according to a furtherembodiment, be reversed in polarity such that the gate (G) and drain (D)were connected to the positive rail 303 a and the source (S) connectedto the negative rail 303 b.

A consequence of using the single FET 501 configured in this manner toprovide the NDR 302, is that the gate capacitance of the FET 501 isparasitic to the driver circuit 102 because it is connected to an outputrail of the driver 301, thus potentially reducing the operatingbandwidth.

FIG. 6 shows an embodiment in which the nonlinear differential resistorNDR is implemented by means of two back to back first and second FETtransistors 601, 602 (Q1 and Q2) connected in series. The drain (D) ofthe first FET 601 is connected to the positive rail 301 a (D_OUT+) andthe drain (D) of the second FET 602 being connected to the negative rail301 b (D_OUT−). The gate (G) and source (S) terminals of the first andsecond FETs 601, 602 are connected together. Thus, both the first andsecond FETs act as two terminal devices and because they are in seriesact as a single non-linear resistor. This two-transistor embodiment isadvantageous over the FIG. 5 embodiment, for example, because the gateterminals of the neither FET 601 or FET 602 are connected to outputterminals of the driver 301, thus avoiding introducing parasiticcapacitances which may reduce operating bandwidth.

An alternative embodiment is shown FIG. 7, where the drain (D) andsource (S) terminals of both first and second FETs 601, 602 (Q1 and Q2)implementing the nonlinear resistor are inverted such that the source(S) terminal of the first FET 601 (Q1) is connected to the positivedriver rail 301 a (D_OUT+) and the source (S) terminal of the second FET602 (Q2) is connected to the negative driver rail 301 b (D_OUT−). Thegate (G) and drain (D) of the first and second FETs 601, 602 areconnected together. This is possible, for example, where the polarity ofthe FET in two-terminal configuration is not important due tosymmetrical electrical characteristics.

An alternative embodiment is shown in FIG. 8, where the FETs 601 and 602have the same polarity such that the drain (D) of the first FET 601 (Q1)is connected to the positive rail 301 a (D_OUT+) and the source (S) ofthe second FET 602 (Q2) is connected to the negative rail 301 b(D_OUT−). The gates (G) of the first and second FETs 601, 602 areconnected with the source of the first FET 601 (Q1) and the drain (D) ofthe second FET 602 (Q2) In this embodiment the terminals of the secondFET 602 (Q2) are inverted when compared to the embodiment shown in FIG.6, but according to another embodiment, the same result can be achievedif the terminals of the first FET 601 (Q1) are inverted while theterminals of Q2 are left as they are with respect to the FIG. 6embodiment.

A further embodiment is shown in FIG. 9, which is similar to theembodiment of FIG. 6, however, two resistors 901, 902 (R1 and R2) areinserted between the connection terminals 302 a, 302 b of the NDR andthe respective output rails 301 a, 301 b of the driver 301. Theresistors can be of the same value or different value. The resistors901, 902 (R1 and R2) act to effectively fix the amount of currentflowing through the circuit and thus protect the circuit elements (e.g.the FETs 601, 602) from damage due to electrostatic discharge. Suchdischarge may occur, for example, when the driver amplifier circuit isbeing mounted or measured due to electrostatic discharge from ameasurement or installation instrument. The resistance value of the NDRmay be low which means that a high and damaging current may flow throughthe circuit in the case of such an electrostatic discharge. Thelinearity of the resistor(s) 901, 902 may reduce the THD cancellationbenefit because it reduces the non-linearity of the NDR to some extent,but by careful choice of resistor 901, 902 values this can be managed sothat a substantial benefit in THD reduction is gained while stillprotecting the circuit 102 from electrostatic discharge. Although theNDR 302 in FIG. 9 is shown as the two FET 601, 602 configurationaccording to the embodiment of FIG. 6, it will be appreciated that theprotection resistors 901, 902 (R1 and R2) may be used with otherimplementations of the NDR e.g. the single transistor implantation ofFIG. 5.

In the above embodiments, the NDR is comprised of one or more FETs suchas n-type or p-type MOSFETs. However, as will be appreciated, othertransistor elements could be used instead. For example, bipolar junctiontransistors could be used or junction field effect transistors providedthe increasing resistance with voltage behaviour could be provided i.e.by configuring the transistor (or other circuit element) as a non-lineartwo terminal device.

A person of ordinary skill in the art may be aware that, in combinationwith the examples described in the embodiments disclosed in thisspecification, units and algorithm steps may be implemented byelectronic hardware, or a combination of computer software andelectronic hardware. Whether the functions are performed by hardware orsoftware depends on particular applications and design constraintconditions of the technical solutions. A person skilled in the art mayuse different methods to implement the described functions for eachparticular application, but it should not be considered that suchimplementation goes beyond the scope of the present invention.

It may be clearly understood by a person skilled in the art that, forthe purpose of convenient and brief description, for a detailed workingprocess of the foregoing system, apparatus, and unit, reference may bemade to the corresponding process in the foregoing method embodiments,and details are not described herein again.

In the embodiments provided in the present application, it should beunderstood that the disclosed system, apparatus, and method may beimplemented in other manners. For example, the described apparatusembodiment is merely exemplary. For example, the unit division is merelylogical function division and may be other division in actualimplementation. For example, a plurality of units or components may becombined or integrated into another system, or some features may beignored or not performed. In addition, the displayed or discussed mutualcouplings or direct couplings or communications connections may beimplemented through some interfaces. The indirect couplings orcommunications connections between the apparatuses or units may beimplemented in electronic, mechanical, or other forms.

The units described as separate parts may or may not be physicallyseparate, and parts displayed as units may or may not be physical units,may be located in one position, or may be distributed on a plurality ofnetwork units. A part or all of the units may be selected according toactual needs to achieve the objectives of the solutions of theembodiments.

In addition, functional units in the embodiments of the presentinvention may be integrated into one processing unit, or each of theunits may exist alone physically, or two or more units are integratedinto one unit.

When the functions are implemented in the form of a software functionalunit and sold or used as an independent product, the functions may bestored in a computer-readable storage medium. Based on such anunderstanding, the technical solutions of the present inventionessentially, or the part contributing to the prior art, or a part of thetechnical solutions may be implemented in the form of a softwareproduct. The computer software product is stored in a storage medium,and includes several instructions for instructing a computer device(which may be a personal computer, a server, or a network device) toperform all or a part of the steps of the methods described in theembodiments of the present invention. The foregoing storage mediumincludes: any medium that can store program codes, such as a USB flashdrive, a removable hard disk, a read-only memory (ROM, Read-OnlyMemory), a random access memory (RAM, Random Access Memory), a magneticdisk, or an optical disc.

The present inventions can be embodied in other specific apparatusand/or methods. The described embodiments are to be considered in allrespects as illustrative and not restrictive. In particular, the scopeof the invention is indicated by the appended claims rather than by thedescription and figures herein. All changes that come within the meaningand range of equivalency of the claims are to be embraced within theirscope.

What is claimed is:
 1. A driver amplifier circuit comprising: anon-linear differential amplifier; and a non-linear resistor connectedacross output terminals of the differential amplifier, wherein thenon-linear resistor has a resistance value that increases as thedifferential voltage amplitude across the non-linear resistor increases.2. The circuit according to claim 1, wherein the non-linear resistor isconfigured to absorb a current that depends on voltage according to ahyperbolic tangent function.
 3. The circuit according to claim 1,wherein the non-linear resistor is configured so that the currentflowing across the non-linear resistor is subtracted from the outputcurrent of the non-linear differential amplifier to provide a current atthe output of the driver amplifier circuit that is substantially linearwith respect to an input voltage provided at inputs of the non-lineardifferential amplifier.
 4. The circuit according to claim 1, wherein thenon-linear resistor comprises at least a first transistor configured tooperate as a two-terminal device.
 5. The circuit according to claim 4,wherein the non-linear resistor further comprises a second transistorconfigured as a two-terminal device that is connected in series with thefirst transistor.
 6. The circuit according to claim 5, wherein the firstand second transistors are field effect transistors.
 7. The circuitaccording to claim 6, wherein a gate of the first transistor is coupledto a gate terminal of the second transistor.
 8. The circuit according toclaim 7, wherein the drain terminal of the first transistor is coupledto a positive output terminal of the differential amplifier; the drainterminal of the second resistor is coupled to a negative output terminalof the differential amplifier; and the source terminal of the first andsecond transistors is coupled to the gates of the first and secondtransistors.
 9. The circuit according to claim 7, wherein the sourceterminal of the first transistor is coupled to a positive outputterminal of the differential amplifier; the source terminal of thesecond resistor is coupled to a negative output terminal of thedifferential amplifier; and the drain terminal of the first and secondtransistors is coupled to the gates of the first and second transistors.10. The circuit according to claim 7, wherein the drain terminal of thefirst transistor is coupled to a positive output terminal of thedifferential amplifier; the source of the second resistor is coupled toa negative output terminal of the differential amplifier; and the drainterminal of the first transistor and the source terminal of the secondtransistor are coupled with the gates of the first and secondtransistors.
 11. The circuit according to claim 7, further comprising alinear resistor connected between the non-linear resistor and thepositive and/or negative output terminals of the differential amplifier.12. A transmitter comprising a driver amplifier circuit according toclaim
 1. 13. The transmitter according to claim 12, further comprisingan electro-optical modulator configured to modulate the output of thedriver amplifier circuit.
 14. The transmitter according to claim 12,further comprising a signal source configured to provide a signal to thedriver amplifier circuit.
 15. The transmitter according to claim 14,wherein the signal source is configured to provide a signal modulatedaccording to a quadrature amplitude modulation scheme.
 16. Thetransmitter according to claim 14, wherein the signal source comprises adigital-to-analogue converter.